As one of the properties required for the tread of tires, cut resistance is given. This property is especially important when tires are used on roads in bad condition, construction sites or the like which are likely to give visible injuries to tires. As a direction toward the improvement of the cut resistance of tread, to improve the hardness of rubber composition and, at the same time, to make the elongation at break larger have been believed to be effective. For improving the hardness of rubber composition, techniques may be considered in which the crosslinking density is increased by mixing carbon black at a high ratio, increasing the content of sulfur, or the like.
According to these techniques, however, the elongation at break is decreased and there occurs a phenomenon called chipping, i.e., rubber chips come off from a tire. In order to improve this property, various tests such as the use of a thermoplastic resin, a thermosetting resin, etc. have been made. However, these attempts often have not been able to achieve a desirable result in other properties, particularly in heat resistance and heat build-up. Thus, sufficient effects have not necessarily been obtained. For example, as seen in Japanese Unexamined Patent Publication No. 48-38338, the compatibility of cut resistance and heat build-up was achieved, but durability and heat resistance were not sufficient.
Not only in the tread of tires but also in other portions of tires, it is important to allow high hardness, heat resistance and low heat build-up to coexist. However, it is an extremely difficult assignment to achieve.
In rubber products other than tires, the coexistence of high hardness and low heat build-up is also required, for example, for rubber vibration insulators, particularly, a rubber for the suspension of vehicles.
As specific examples of pneumatic tires in which polyethylene is mixed, U.S. Pat. No. 4,675,349 and U.S. Pat. No. 5,341,863 are given. The former is characterized by mixing a polyethylene having a softening point of 35.degree. C. or above at a temperature lower than that. In this case, inevitably, the polyethylene should be added in the form of fine particles. Such a polyethylene is difficult to handle at the time of mixing. Furthermore, when mixed, polyethylene particles may aggregate to thereby worsen the physical properties of the resultant compound. The latter is characterized by using LDPE (low density polyethylene) of which the melting point of the crystal falls within a range of 104-115.degree. C. In this case, changes in the physical properties of the resultant rubber composition are drastic at a high temperature, as described later in the description of the present invention. Therefore, the latter composition should be said difficult to use, in particular, as a rubber composition for tires.
Japanese Unexamined Patent Publication No. 7-266454 discloses a pneumatic tire in which LDPE or LLDPE (linear low density polyethylene) is mixed. In this case, changes in the physical properties of the resultant rubber composition are also drastic at a high temperature, and this composition should be said difficult to use, in particular, as a rubber composition for tires.
Generally, when a polyethylene having a low melting point is used, permanent set in fatigue attributable to the creep of the polyethylene is observed in the resultant rubber composition in addition to the above-mentioned problems. Thus, the use of such a polyethylene is not appropriate.